A broadcasting frame rate defined by NTSC (National Television Standards Committee), a standard of analog television system, is 29.97 fps (frames per second) interlaced. Hereafter this frame rate is denoted as “30 fps” or “60i” to simplify description. Natural movement of moving images is provided if the frame rate upon capturing a moving image and the frame rate upon broadcasting match.
The moving image materials to be broadcasted include movies and animation. The moving image capturing frame rate of movies and animation is normally 24 fps progressive format. And this is denoted as “24p” herein below. To broadcast 24p moving image materials according to the NTSC standard, 2-3 pulldown processing is performed for converting 24p into 60i.
FIG. 1 is a diagram depicting an example of the 2-3 pulldown processing. A frame rate upon capturing moving image materials is called “source rate”, a frame rate upon broadcasting is called “broadcasting rate”, and a frame rate upon replaying moving images in a replay apparatus is called “display rate”. In NTSC, one frame consists of two fields: an odd field and an even field. A field in which only odd scan lines are drawn, out of all the scan lines of one frame, is called an “odd field”. A field in which only even scan lines are drawn, out of all the scan lines of one frame, is called an “even field”.
An example of the 2-3 pulldown processing will be described below. FIG. 1 indicates a moving image material of which source rate is 24 fps, and the moving image material includes a frame F1 to a frame F4. To transmit the frame F1 to the frame F4 at 60i, two fields, that is an odd field F1a and an even field F1b, are generated from the frame F1, which are transmitted in the sequence of the odd field F1a and the even field F1b. After transmission of the frame F1, an odd field F2a and an even field F2b are generated from the frame F2, which are transmitted in the sequence of the odd field F2a and the even field F2b and the odd field F2a. The same processing for frame F1 and frame F2 is also executed for the subsequent frames, frame F3 and frame F4. In other words, an odd field F3a and an even field F3b generated from the frame F3 are transmitted in this sequence, then an odd field F4a, an even field F4b and an odd field F4a, generated from the frame F4, are transmitted in this sequence. Thus according to the 2-3 pulldown processing, a processing for transmitting two fields for one frame and a processing for transmitting three fields for one frame are alternately repeated for a plurality of 24 fps frames. In the processing for transmitting three fields, an odd field having identical content is transmitted twice.
According to this 2-3 pulldown processing, 60 fields can be generated and transmitted from 24 frames, so as to match the display rate (60p) of a TV receiver and the frame rate of broadcasting. As a result, moving images of which source rate is 24 fps can be broadcasted at 60i. Hereafter “converting a frame rate” or “a frame rate conversion” refers to executing such a processing as the 2-3 pulldown processing on moving image materials, so as to convert a frame rate into a frame rate that is different from the source rate.
If an image frame of which frame rate is converted into 60i is input to a progressive scan type display apparatus which displays images at 60p, for example, a pulldown cancellation is performed in the display apparatus, so that the original frames are restored. In other words, if two fields are received for one frame (e.g. frames F1 and F3 in FIG. 1), the display apparatus combines the received two fields (that is, an odd field and an even field) to generate two original frames. For example two F1 frames are generated from the odd field F1a and the even field F1b. If three fields are received for one frame (e.g. frames F2 and F4 in FIG. 1), on the other hand, the display apparatus generates three original frames from the received three fields (two odd fields and one even field). For example, three F2 frames are generated from the two odd fields F2a and the even field F2b. 
The display apparatus draws the generated original frames on screen according to the display time axis (display rate) of 60p. As a result, the processing of displaying the 24 fps frames, such as the frames F1 and F2 twice, and the processing of displaying 24 fps frames, such as the frames F2 and F4 three times, are executed alternately on the display time axis (see FIG. 1).
Recently one segment partial reception service for portable telephones and mobile terminals began. The one segment partial reception service for portable telephones and mobile terminals is normally called “One Seg”, which is a terrestrial digital broadcasting service that enables TV programs to be played on mobile phones, car navigation systems and other portable devices. The frame rate of One Seg, that is a broadcasting rate thereof, is about 15 fps. So the broadcasting rate of One Seg is hereafter regarded as 15 fps to simplify description.
Thus the broadcasting rate of One Seg is lower than the frame rage of current analog TV broadcasting (60i) and frame rate of 24 fps moving image materials. In the case of display moving images which are input at a broadcasting rate that is lower than the source rate, such as a case of broadcasting 24 fps moving image materials in One Seg, the following problems occur.
FIG. 2 illustrates an example of processing for converting the frame rate 24 fps into the frame rate 15 fps. In the case of the example in FIG. 2, eight frames (frames A to I in FIG. 2) corresponding to ⅓ second at 24 fps respectively are converted into five frames corresponding to ⅓ seconds at 15 fps. In this example, three frames, such as frames C, E and H, are skipped from the original eight frames. Therefore, there is likelihood that the motion of the moving image becomes unnatural.
In order to solve the unnatural motion of moving images, a technology to insert intermediate frames among frames is available. The processing for inserting intermediate frames among frames is called “frame interpolation”.
By interpolating and displaying intermediate frames, which correspond to the frames skipped during converting frame rates, among the images which were input at 15 fps, moving images having natural motion are displayed in the display apparatus. However, in the case of displaying moving image material of which source rate is 24 fps in One Seg of which broadcasting rate is 15 fps, the skipped sections of frames upon frame the rate conversion cannot be specified. In other words, in a case of displaying moving image material of which source rate is 24 fps in One Seg of which broadcasting rate is 15 fps, frames are not appropriately interpolated, and unnatural motion can be generated in moving images.    [Patent document 1] Japanese Patent Application Laid-Open No. 2008-028850    [Patent document 2] Japanese Translation of PCT Application No. H11-513541